In general, memories can be divided into two major types. One is the volatile memory; the other is the nonvolatile memory. The difference between the two is that when the power is broken, the data stored in a volatile memory will disappear, whereas that in a nonvolatile memory won't. Once the power is recovered, the data stored in a nonvolatile memory can be accessed.
Volatile memories include dynamic random access memories (DRAM) and static random access memories (SRAM). Their advantages include fast access and low price. On the other hand, nonvolatile memories according to the prior art include read only memories (ROM) and flash memories. The USB disks normally seen adopt flash memories. The main technology is the NAND technology, which uses floating-gate transistors to store data. According to the quantity of electrons stored in the semiconductor oxide layer or metal layer, the signals of 0 and 1 can be discriminated. The drawbacks of the technology include high operating voltages, low speed, and degraded data retention due to thinning of the tunneling oxide in the process of device miniaturization.
Accordingly, in order to maintain the advantages of current memories and improve the problems of flash memories, scientists are devoted in developing novel nonvolatile memories. Presently, novel nonvolatile memories can be classified into four types: ferroelectric RAM (FERAM), magnetoresistive RAM (MRAM), phase-change RAM (PCRAM), and resistive RAM (RRAM).
Among the memories, RRAM is the simplest in terms of structure. It usually adopts low-activity hard-to-oxidize metals, such as Pt and TiN, as the top and bottom electrodes; metal oxides, such as NiO, TiO2, HfO2, Ta2O5, ZrO2, Al2O3 are mostly adopted as the middle oxide layer. The resistance transformation mechanism of an RRAM depends on the movement of oxygen ions in the middle oxide layer. By applying a bias on the top electrode, the oxygen ions will move under the action of the electric field. After they move away, the original sites will form oxygen vacancies, which can be used as the path for electron movement. Alternatively, a highly conductive anaerobic phase can be formed in the electrolyte. The oxygen vacancies continue to accumulate, leading to the formation of conductive channels. The device transforms from the high-resistive state to the low-resistive state. Then a reverse bias is applied, which changes the moving direction of the oxygen ions and enables the oxygen ions to recover the oxygen vacancies. Consequently, the conductive channels are broken, and the device transforms from the low-resistive state to the high-resistive state. RRAM owns the property of bipolar transformation. The concentration of oxygen vacancies determines the electrical characteristics of such devices. If the concentration is too low, devices will become unstable, and the endurance will degrade as well.